1. Field of the Invention
The present invention relates to an antenna system.
2. Background of the Invention
An antenna is the heart of a wireless communications system. Antennas in transmitters convert electrical signals into airborne radio frequency (RF) waves, and in receivers they convert airborne waves into electrical signals. Without antennas there are no wireless communications.
The size of an antenna depends on the radio frequency for which the antenna is designed. The higher the frequency, the smaller the antenna. Therefore, wireless telephones use small antennas to communicate at high frequencies. Because there is a finite range of high frequencies that is allocated for wireless communications, a wireless service provider must reuse some or all of its allocated frequencies to increase call handling capacity, i.e., to enable more customers to use their wireless telephones at the same time in the same service area.
To reuse frequencies, a wireless service provider divides its service area into “cells,” and it equips each of the cells with a low-powered antenna system. Antenna systems in two non-adjacent cells may use the same frequency. Cells generally fall into three categories: “macrocells,” “microcells,” and “picocells.” A macrocell covers a relatively large area (e.g., about 50-mile radius), and it is optimized to serve users who are highly mobile such as those in automobiles. A microcell covers a smaller area (e.g., about 10-mile radius), and it is optimized for wireless device users who are less mobile such as pedestrians. A picocell covers an even smaller area (e.g., a tunnel or a parking garage). The antenna system for a picocell requires extremely low output power but it can direct cellular signal into an isolated spot such as a low-lying, tree-covered road intersection.
An antenna system at each picocell typically has a donor antenna, a signal-processing device such as an amplifier (for analog signals) or a repeater (for digital signals), and a coverage antenna. These three components are serially connected by coaxial cables. The components are typically mounted on a utility pole that is about 40 to 50 feet tall. The donor antenna receives downlink signals from a macrocell site (also known as the donor cell site) and channels the downlink signals to the signal-processing device. The signal-processing device either amplifies or repeats the downlink signals before the coverage antenna broadcasts the downlink signals to the vicinity of the picocell. Similarly, the coverage antenna receives uplink signals from the vicinity of the picocell and the donor antenna re-transmits the uplink signals to the macrocell site after the amplifier or the repeater has processed the uplink signals. The donor antenna is typically a directional antenna that has a clear line of sight to the donor cell site. On the other hand, the coverage antenna is typically an omnidirectional antenna that has a 360-degree “view” of the picocell. To maximize signal reception and coverage, both antennas must be mounted as high as possible.
Each of the donor and coverage antennas has its own RF patterns that are often known as side lobes. The side lobes of the donor antenna often overlap with the side lobes of the coverage antenna, resulting in a signal looping effect. As a result, the signal-processing device is often saturated by signals looping between the two antennas. The saturation situation causes the antenna system to shut down.
One solution to reduce the looping effect is to separate the donor antenna from the coverage antenna as far as possible. However, the existing antenna technology still does not offer a satisfactory solution to the looping effect due to the following constraints. First, the antennas cannot be separated more than twenty feet apart on a utility pole that is about 40 to 50 feet high. Second, existing antennas are bulky and heavy, making them difficult to mount at higher locations. Third, existing antennas have large cross-sections that are not desirable at higher altitudes due to wind loading. Fourth, extending the height of the utility pole is not desirable due to cost, environmental, and aesthetic concerns.